COMMITMENTS AND CONTINGENCIES - Long-Term Contractual Arrangement Narrative (Details) $ in Millions				12 Months Ended
	Nov. 22, 2019	Mar. 15, 2019	Dec. 28, 2018 MWh	Dec. 31, 2025 USD ($) MWh contract MW
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 22,514.7
Peaker Contracts For Differences
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 182.1
Percentage of costs and benefits borne by United Illuminated Company				20.00%
Period of payment to generation facility owner				30 years
NSTAR Electric
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 12,723.1
CL&P
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				7,016.4
CL&P | Peaker Contracts For Differences
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 182.1
Number of capacity contracts | contract				3
Amount of peaking capacity contracts (in MW) | MW				500
Percentage of costs and benefits borne				80.00%
CL&P, NSTAR Electric and PSNH | Transmission Support Commitments
Long-term Purchase Commitment [Line Items]
Period of payment of annual operation and maintenance expense				20 years
Hydro-Québec
Long-term Purchase Commitment [Line Items]
Annual purchase requirement (in MWh) | MWh				1,200
Hydro-Québec | NSTAR Electric
Long-term Purchase Commitment [Line Items]
Contract term (in years)				20 years
Contract term (in years)				20 years
Annual purchase requirement (in MWh) | MWh				579
Hydro-Québec | NSTAR Electric | Construction in Progress
Long-term Purchase Commitment [Line Items]
Total construction contract cost				$ 6,700.0
Hydro-Québec | NSTAR Electric | Minimum
Long-term Purchase Commitment [Line Items]
Estimated costs under construction contract				260.0
Hydro-Québec | NSTAR Electric | Maximum
Long-term Purchase Commitment [Line Items]
Estimated costs under construction contract				$ 420.0
Hydro-Québec | NECEC Transmission Service Agreement
Long-term Purchase Commitment [Line Items]
Contract term (in years)				20 years
Hydro-Québec | NECEC Transmission Service Agreement | Construction in Progress
Long-term Purchase Commitment [Line Items]
Total construction contract cost				$ 2,300.0
Hydro-Québec | NECEC Transmission Service Agreement | Minimum
Long-term Purchase Commitment [Line Items]
Estimated costs under construction contract				95.0
Hydro-Québec | NECEC Transmission Service Agreement | Maximum
Long-term Purchase Commitment [Line Items]
Estimated costs under construction contract				$ 140.0
Vineyard Wind Project | NSTAR Electric
Long-term Purchase Commitment [Line Items]
Contract term (in years)				20 years
Contract term (in years)				20 years
Annual purchase requirement (in MWh) | MWh				800
Vineyard Wind Project | NSTAR Electric | Construction in Progress
Long-term Purchase Commitment [Line Items]
Total construction contract cost				$ 3,200.0
Vineyard Wind Project | NSTAR Electric | Minimum
Long-term Purchase Commitment [Line Items]
Estimated costs under construction contract				100.0
Vineyard Wind Project | NSTAR Electric | Maximum
Long-term Purchase Commitment [Line Items]
Estimated costs under construction contract				200.0
Millstone Nuclear Power Station and Seabrook Nuclear Power Plant
Long-term Purchase Commitment [Line Items]
Estimated cost of energy purchase contracts				$ 1,600.0
Milestone Nuclear Power Station
Long-term Purchase Commitment [Line Items]
Annual purchase requirement (in MWh) | MWh			9,000,000
Percent of the facility's output required to be purchased		50.00%
Milestone Nuclear Power Station | CL&P
Long-term Purchase Commitment [Line Items]
Contract term (in years)			10 years
Percent of the facility's output required to be purchased		40.00%
Milestone Nuclear Power Station | UI
Long-term Purchase Commitment [Line Items]
Contract term (in years)			10 years
Seabrook Nuclear Power Plant
Long-term Purchase Commitment [Line Items]
Percent of the facility's output required to be purchased	18.00%
Seabrook Nuclear Power Plant | CL&P
Long-term Purchase Commitment [Line Items]
Contract term (in years)			8 years
Percent of the facility's output required to be purchased	15.00%
Seabrook Nuclear Power Plant | UI
Long-term Purchase Commitment [Line Items]
Contract term (in years)			8 years